Gary A. Haugen has written a book that is a combination of theological insight, moving stories, and creative solutions regarding the issue of injustice in the world.
Good News About Injustice: A Witness of Courage in a Hurting World (IVP, 1999) is well worth reading on this issue. One poignant section deals with how God brings justice to those who need it.
"If God seeks justice for the oppressed, how exactly does he do that? If, as Psalm 10 says, Gods helps the fatherless, defends the oppressed, 'breaks the arm' of evil people and calls them to account for their wickedness--how does he do this?...
"A fair question. Do we have an answer? I think we do, and it has everything to do with you and me.
"Unless the work of seeking justice is a category of endeavor that is completely different from every other activity on earth that is important to God, the answer to the how question has something to do with what God's people do or don't do. If you think about it, two truths apply to everything that God wants accomplished on earth: (1) he could accomplish it on his own through supernatural power; but instead, (2) he chooses for the most part to limit himself to accomplishing that which he can achieve through the obedience of his people." (pp. 96-97)
So even as we cry out for justice we should also be praying and searching to see in what manner we might be the answer to that cry for justice.